A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, the patterned substrate is inspected and one or more parameters of the patterned substrate are measured. The one or more parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and/or critical linewidth of developed photosensitive resist. This measurement may be performed on a target of the product substrate itself and/or on a dedicated metrology target provided on the substrate. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of a scanning electron microscope and/or various specialized tools.
A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing one or more properties of the beam before and after it has been reflected or scattered by the substrate, one or more properties of the substrate can be determined. Two main types of scatterometer are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angle resolved scatterometer uses a relatively narrowband radiation beam and measures the intensity of the scattered radiation as a function of angle.
A particular application of scatterometry is in the measurement of feature asymmetry within a periodic target. This can be used as a measure of overlay error, for example, but other applications are also known. In an angle resolved scatterometer, asymmetry can be measured by comparing opposite parts of the diffraction spectrum (for example, comparing the −1st and +1st orders in the diffraction spectrum of a periodic grating). This can be done simply in angle-resolved scatterometry, as is described for example in U.S. patent application publication US2006-066855.